AC induction motors are normally used for applications which do not require variable speed, while DC motors are more common in applications where speed control is required.
AC induction motors have a simpler construction than DC motors, and they are less susceptible to environmental influence. Especially the fact that commutator and brushes are not used is of great advantage. Thus, for a long time there has been a need to replace DC motors by AC induction motors. In order to make this possible there is required a control system for an AC induction motor. Today many different control systems for AC induction motors are available on the market.
Most commercially available control systems generate control signals in a relatively simple way. The amplitude and frequency of the motor control signals are generated as a certain, invariable function of the input signal. It is possible to show, that such control systems operate well at high motor speeds, but unsatisfactory at motor speeds close to zero.
The expensive parts of a motor control system are the "power amplifiers" (e.g. thyristor frequency inverters which transform the fixed voltage and frequency of the AC power line into variable voltage and frequency). The "signal circuits" for generation of the control signals operate at low power, and their cost is almost negligible in comparison with the power amplifiers. Thus, it is of great technical and commercial interest to create better control systems with help of better "signal circuits". Also improvement of details are important. As a basis for the design of the "signal circuits" an optimal control theory or strategy must be developed. The "signal circuits" may then be realized using any technical means, e.g. digital or analog electronic circuits.
A general control systems for AC induction motors should permit four quadrant operation, that is active drive and brake in both directions of rotation. Furthermore, full torque should be available at all speeds, including zero speed. It should be possible to operate the motor in so-called open control systems (without feedback) as well as in so-called closed control systems (with feedback). In the latter case the dynamic performance of the control system is essential. The motor must react on the input control signal without unnecessary time delay. An optimally constructed control system gives the AC induction motor a dynamic performance comparable to that of the DC-motor, and this possibility should be made use of.
Among existing AC motor control systems it is possible to distinguish between two principally different control methods. According to the first method, here named "Type I", the motor is considered as a separate component. It is controlled by externally generated control signals. According to the second method, here named "Type II", the motor is an active part of the control system. Measured signals from the motor itself are fed back to the control system and influence the generation of control signals.
From a theoretical point of view "Type II" should be the best control system. Common for all types of modern process control is the requirement to collect information about the process "state" by measuring all "state variables". Knowing the "state" of the process it is possible to generate optimal control signals. However, it is difficult to obtain suitable measuring signals from the motor, especially from a standard motor. For this reason, control systems of "Type II" are uncommon.
Control systems of "Type I" may give good results, if the theoretical motor model, which is the basis for the control system, is correct, and if external disturbances are taken into account. If the operating range of the control systems is reduced, for example by excluding speeds close to zero, a very simple control system may give good results. The simplest system, operating with constant speed on the fixed voltage and frequency of the power line, is known to work excellently.
The Swedish Pat. No. 334,671 discloses a method according to "Type I" which improves the control possibility at low speeds, including zero speed. Especially the response time on control signals is shortened, which makes the AC induction motor suitable for closed loop control systems. This control system will be described in detail later, and some drawbacks will be shown.